In recent years, there have been actively researched or studied capacitive, ultrasonic transducers using micromachining processes (CMUT; Capacitive Micromachined Ultrasonic Transducer). Hereinafter, such a capacitive ultrasonic transducer is referred to as a CMUT. According to such a CMUT, there can be easily obtained a broadband characteristic that is excellent both in a liquid and in an air by transmitting and receiving an ultrasonic wave by the use of a vibration membrane. Therefore, with ultrasonic diagnostics using this CMUT, it becomes possible to make an ultrasonic diagnosis with higher precision than with a conventional medical diagnostic modality, and hence ultrasonic diagnostics is being noted as a promising technology in these days.
This CMUT has a construction in which a vibration membrane provided with an upper electrode and a substrate provided with a lower electrode are arranged in opposition to each other, and the vibration membrane is supported by a support member so as to form a gap between the vibration membrane and the substrate (see Japanese patent application laid-open No. 2006-319712). When this is driven to operate, an electrostatic attraction force is first generated between both the electrodes by applying a DC voltage to the lower electrode, so that the vibration membrane is thereby caused to deform. In addition, by superimposing a fine AC voltage thereon, the vibration membrane is vibrated to oscillate an ultrasonic wave. When the ultrasonic wave is received, the vibration membrane is caused to deform by reception of the ultrasonic wave, whereby the interval or distance between both the electrodes is changed, and a resultant change in the capacitance between both the electrodes is detected as a signal.
In order to enhance the mechano-electric transducing characteristic, it is desirable to decrease the interelectrode interval between the upper electrode provided at a vibration membrane side and the lower electrode provided at a substrate side. Therefore, by applying a high DC voltage, the vibration membrane can be deformed more greatly so as to make the above-mentioned interelectrode interval narrow. However, the application of such a high voltage also poses a problem in putting the formation of a surface insulation film on the transducer into practical use so as to avoid resultant adverse effects. In case where the CMUT with such a high voltage applied thereto is used for acoustic diagnostics, an unfavorable influence might be caused on human bodies.
In the past, as an example in which an interelectrode interval is made narrow by application of a low voltage, U.S. Pat. No. 6,426,582 discloses a CMUT which will be described below. In this U.S. Pat. No. 6,426,582, a vibration membrane is caused to deform downward, and in such a deformed state, a resist resin is heated and coated around the vibration membrane. Thereafter, the resist is cooled to harden, and the vibration membrane is fixed in its periphery with its shape being naturally deformed in a downward direction, whereby an interval between capacitive electrodes is formed to be small. In addition, this U.S. Pat. No. 6,426,582 adopts a construction in which the interelectrode interval is controlled by protrusions. That is, the construction adopted is such that the protrusions are formed on a lower side of the vibration membrane, and they alone are in contact with an underlayer substrate, with the central portion of the vibration membrane being not in contact with the underlayer.
On the other hand, in recent years, a collapse mode has being noted as a new operation mode different from a conventional mode that is an ordinary operation mode in a CMUT. This collapse mode means an operation mode in which when a DC voltage is applied to a lower electrode, a vibration membrane is attracted to an underlayer electrode under the action of a DC electrostatic force thereof, so that the vibration membrane is thereby made into a collapsed or crushed state in which it is caused to operate while being in contact with the lower electrode. In addition, this specific voltage is called a collapse voltage.
In this collapse mode, it is said that sensitivity and driving ability are higher than the above-mentioned conventional mode (see IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, Vol. 52, No. 2, February 2005, p. 326-339). In this collapse mode, unlike the conventional, mode in which a gap exists between the vibration membrane and the substrate, there is generated, in part of the vibration membrane including the upper electrode, a region that is in contact with a region of the substrate including the lower electrode. In this state, an ultrasonic wave can be oscillated or emitted by superposing a minute AC voltage to make those portions of the vibration membrane other than the contact region thereof vibrate by means of this minute AC voltage. In addition, an ultrasonic wave can also be received, just like in the above-mentioned conventional mode.
On the other hand, in order to operate the CMUT in the above-mentioned collapse mode, it is necessary to apply an extremely high DC voltage so as to place the vibration membrane into contact with the lower electrode. The DC voltage (collapse voltage) needed here is in the range of from about 130 to 150 V, and the CMUT can not be kept operating in this mode when such a voltage can not be provided. However, it is extremely difficult to put a circuit operating with such a high voltage to practical use, and in case where the MDT, being operated with such a high voltage, is used for acoustic diagnostics, unfavorable influences might be exerted on human bodies. Moreover, if such a high voltage is applied, the vibration membrane might cause dielectric breakdown, thereby making the lower electrode and the upper electrode be short-circuited to each other.
In the past, in Japanese patent application laid-open No. 2005-27186, there has been proposed a CMUT which is constructed in the following manner so as to decrease a DC voltage in a collapse mode. In Japanese patent application laid-open No. 2005-27186, a construction is used in which a vibration membrane is attracted with the use of a magnet. Specifically, a part of the vibration membrane including a magnetic material is attracted by a magnetic field from the outside, whereby an interval between capacitive electrodes is decreased, as a result of which a high DC voltage (collapse voltage) is made unnecessary, thus lowering a required voltage.
In addition, in Japanese patent application laid-open No. 2006-50314, a construction is adopted in which a vibration membrane is electrified by a corona discharge treatment, thereby making a high DC voltage (collapse voltage) unnecessary.
As stated above, in order to operate a CMUT in a collapse mode, a high DC voltage of about 130-150 V (collapse voltage) is needed as a DC voltage (collapse voltage). Therefore, as referred to above, there arise problems such as circuit construction, influences on human bodies, a short-circuit between a lower electrode and an upper electrode, etc.
Further, even in the above-mentioned examples that have been proposed for coping with these problems, the following unfavorable influences are given to the vibration mass, the rigidity, the stability, etc., of the vibration membrane.
For instance, in Japanese patent application laid-open No. 2005-27186 in which the lowering of the voltage is intended by the vibration membrane being attracted with the use of the magnet, not only deposition and magnetization of a magnetic material on an upper portion or an internal, portion or a lower portion) of the vibration membrane become necessary, but also a magnetic field forming means is required for the underlayer substrate, resulting in a complicated structure. In addition, there is also a problem in that an amount of initial displacement of the vibration membrane is attracted by the magnetic field, and hence is liable to be influenced by external magnetic fields and external disturbances.
Also, in Japanese patent application laid-open No. 2006-50314 in which a vibration membrane is electrified by a corona discharge treatment, there are the following problems. That is, the amount of electrification by an electrical discharge is liable to be influenced by environmental factors such as humidity, dielectric substances, etc., and the amount of electrification in the vibration membrane and the amount of initial displacement thereof are unstable, and variation between elements is large.
In addition, in U.S. Pat. No. 6,426,582 in which the protrusions are formed on the lower side of the vibration membrane, and they alone are in contact with the underlayer substrate, with the central portion of the vibration membrane being not in contact with the underlayer, only a space formed inside the protrusions vibrates, and those portions outside the protrusions are fixedly held against vibration by means of the resist.
Accordingly, this can not be called operating in a collapse mode in a strict meaning, but if this is converted into a collapse mode, there will be the following problems. That is, in case where the deformed shape of the vibration membrane is kept by such hardening of the resist, the shape of the vibration membrane is changed and is made unstable due to a change over time of the resist, and/or a temperature-related change in property or quality thereof. In addition, because the resist covers an outer periphery of the vibration membrane, there is also another problem that an effective area (filling factor) receiving an ultrasonic wave is decreased.
In addition, in the conventional CMUT, the vibration membrane and the substrate are placed in contact with each other as described above in an operating state of the collapse mode, so the variable capacitance between the upper electrode and the lower electrode decreases, resulting in an increase in parasitic capacitance. That is, in a capacitor, which is formed in a region where the vibration membrane and the substrate of both the electrodes are in contact with each other, the distance between the electrodes does not change even at the time when the vibration membrane is caused to vibrate upon transmission and reception of ultrasonic waves, and hence the capacitor does not contribute to the change in capacitance. Due to such an increase in the parasitic capacitance, there arises a problem that the electromechanical transduction efficiency of the CMUT is reduced, and that the signal detection function of the CMUT is lowered.